Organisms, including mammals, invertebrates, and plants have evolved sophisticated and conserved innate immune responses to protect themselves from pathogen attacks. In these systems, host immune responses and disease resistance are achieved by modulation of a large array of genes, but how these processes are regulated is still largely unclear. Small RNAs are non-coding regulatory RNA molecules that control gene expression by mediating mRNA degradation, translational inhibition, or chromatin modification. Our work and those of others have demonstrated a role of host endogenous small RNAs, including microRNAs (miRNAs) and small interfering RNAs (siRNAs), in antibacterial innate immunity. We discovered that several pathogen-induced host endogenous siRNAs, including natural antisense transcripts-derived siRNAs (nat-siRNAs) and long siRNAs (lsiRNAs), facilitate host immune responses by posttranscriptionally silencing the negative regulators of plant immune systems. Epigenetic changes were also observed in bacteria-infected plants, however, the role of small RNA-guided DNA methylation and chromatin modification in plant immunity has not been explored. Using Arabidopsis as a model system and Illumina deep sequencing as a platform, we recently profiled the small RNA population after bacterium-challenge and identified diverse classes of endogenous small RNAs that are regulated by the infection of bacterial pathogens. We hypothesize that these pathogen-responsive small RNAs regulate the expression of genes involved in host immune responses by silencing regulators of host immunity transcriptionally or post-transcriptionally. We propose to investigate the regulation, biogenesis, and function of these pathogen-responsive small RNAs. Specifically, we will perform in depth characterization of several newly identified miRNAs, nat-siRNAs, lsiRNAs, and clustered hc-siRNAs that are particularly regulated by various strains of the bacterium Pseudomonas syringae. Predicted target genes of these small RNAs will be validated and subjected to functional analysis. These targets are likely to be components involved in plant immune response pathways. We will also characterize the hc-siRNAs that directed DNA methylation in response to bacterial infection. We will generate a whole-genome DNA-methylation map before and after bacterial infection, and correlate the siRNAs, DNA methylation, and pathogen-responsive gene expression. The proposed research will significantly advance our understanding of the mechanisms and functions of small RNAs involved in host innate immunity.